1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a structure of a bonding pad for connecting of elements formed on a semiconductor substrate.
2. Description of the Related Art
A portion, which bonds a metal wire to an electrode wiring layer which connects elements formed on a semiconductor substrate to each other in order to extract the electrical characteristics of these elements, has been conventionally called a bonding pad.
FIG. 1 shows an example of a structure of a conventional bonding pad.
As shown in FIG. 1, an insulating oxide film (field oxide film) 12 is formed on an Si substrate 11, and, e.g., a polysilicon layer 13 is formed on the insulating oxide film 12. Thereafter, a bonding pad 14 is formed on the polysilicon layer 13.
The bonding pad 14 has the same structure as that of a normal wiring layer, and is constituted by a barrier metal layer 15 and a metal electrode 16. A passivation film 17 is formed around the layers 15 and 16. A wire 19 is bonded to the metal electrode 16 which constitutes the bonding pad 14 through an opening 18 formed in the passivation film 17. For example, a Ti layer is used as the barrier metal layer 15, and, e.g., an Al or an Al-Si-Cu layer as an Al alloy is used as the metal electrode 16. The reason why the bonding pad 14 is formed on the polysilicon layer 13 is as follows. Since the barrier metal layer 15 such as Ti is hard to be bonded to the BPSG or PSG layer 17, if the barrier metal layer 15 and metal electrode 16 are directly formed on a commonly-used BPSG or PSG layer, they may come off.
With the above structure, however, the mechanical strength of the films between the bonding pad 14 and the Si substrate 11 is insufficient. When the wire 19 is bonded to the metal electrode 16, therefore, a crack 21 may occur in the insulating oxide film 12. For this reason, when a voltage is externally applied to the bonding pad 14 through the wire 19 in order to operate the semiconductor device, a leakage current may flow between the bonding pad 14 and the Si substrate 11.
In addition, the above-described conventional semiconductor device often has a structure wherein a BPSG (boron phosphosilicate glass) film is formed on the insulating oxide film 12. In this structure, if each of the films is thinned as the device is miniaturized, a problem similar to the above problem may occur when a wire is bonded to a bonding pad.